Graziano Pelle had given Saints the lead before Martial struck either side of half-time to turn the game on its head - Juan Mata adding the third and Pelle planting a late header as a consolation.

David De Gea, who almost departed United 24 hours before Martial arrived, was on hand to make two fine saves but it was Martial who had Van Gaal purring afterwards.

"He needs to adapt to the English culture, he is doing great - three matches in a row and three goals," the Dutchman said of his new forward.

"He cannot speak English, I speak French with him. I need help with that from Marouane Fellaini and Morgan Schneiderlin, it is very difficult but he is willing to speak English.

"When you are 19 years old, you cannot expect consistency. Emotionally they shall have a lot of dips so that I expect also from him but that is not a big problem for me.

"I'm very happy that he is in three matches and his talent is at a high level and he adapts to the system of how we want to play, that is also important, not every player can adapt in that system but he shows he wants to do that and he can do that.

"He scores goals. That is the most important thing, that as a striker he scores goals. He can improve I think but he has a high level of talent. He shows it."

Martial's first goal came in controversial fashion as Mata appeared to be offside in the build-up but the France international showed composure ahead of his years to finish coolly past Maarten Stekelenburg.

His second came as he was alert enough to latch on to an under-hit Maya Yoshida back-pass, again seemingly putting his chance away with ease.

Saints boss Ronald Koeman was left fuming by Yoshida's error and ranted about the idea of passing to the goalkeeper in any game.

"In general not criticising Maya, I hate players who hate the ball always back to the goalkeeper, play it into the space in front," he said.

"There were several moments in the game where you put the pressure on them. We saw the weaknesses of Manchester Untied today, defensively.

"I will tell them tomorrow the same story, it is always a risk. We have to learn from that because we made that mistake last year and we did it again today. You can talk about tactics, and goalkeepers, offside or not offside but we lost by ourselves and not by our opponent."